No studies have demonstrated a direct relationship between pregnancy and rib fracture. A case of spontaneous rib fracture in the third trimester presents the opportunity to examine factors unique to pregnancy that may predispose the patient to stress fractures of the lower ribs. A 28-year old woman in week 31 of her pregnancy presented with the chief complaint of acute onset of right upper quadrant pain. A chest radiograph demonstrated a minimally displaced fracture of the right 10th rib. During pregnancy, the enlarging uterus causes certain opposing muscular forces to act on the ribs, making them more susceptible to fracture after minimal trauma or after repeated stresses such as a chronic cough.